Process for controlling the refining of a molten ferrous bath in a basic oxygen furnace



Nov 24, 19% F. A. MIHALOW ETAL PROCESS FOR CONTROLLING THE REFINING OF A MOLTEN FERROUS BATH IN A BASIC OXYGEN FURNACE Filed July 25, 1968 DECAABl/K/ZAT/O/VCl/AVEFOAAA75'Z PROCESS-M/TE/FRl/PffiAT/fCARBO/VCO/WEA/TAAT/O/V A00 A20 I40 INVENTORS fieawbk A/Wi/M/Ow y WWW mf AWGW MWWW 0 United States Patent O 3,542,539 PROCESS FOR CONTROLLING THE REFINING OF A MOLTEN FERROUS BATH IN A BASIC OXYGEN FURNACE Frederick A. Mihalow, Allentown, Fred A. Achey, Ronald J. Fradeneck, and David G. Boltz, Bethlehem, and David W. Kern, Slatedale, Pa.; said Mihalow, Achey, and Fradeneck assignors to Bethlehem Steel Corporation a cor oration of Delaware Filled July 25, 1968, Ser- No. 747,526

Int. Cl. C21c 5/32 US. Cl. 75-60 2 Claims ABSTRACT OF THE. DISCLOSURE A process for controlling the refining of a molten ferrous bath in a basic oxygen furnace in WhlCh ln ection of a preliminarily determined amount of oxygen into the bath is interrupted at a predetermined point to determine the carbon content of the bath after which oxygen injection is resumed and a volume of oxygen, necessary to decrease the carbon content determined at process-interrupt to the final desired carbon content, is determlned by a specific equation and injected into the molten ferrous bath.

BACKGROUND OF THE INVENTION ally injection with gaseous oxygen must-be continued to obtain the desiredfinal carbon content. It is not unusual for this procedure to be repeated two, three and even four times--a-sample of the-bath being obtained after each stoppage of gaseous oxygen. These steps resultinrlosttime in production thereby increasing production costs.

Prior art practicesto control-. the refining ofamolten L ferrous bath in a basic oxygen furnace include. a static charge model control process wherein the charge isthermally balanced before oxygen injection is started, and. a dynamic control system whereingasanalysis, .waste gas mass flow rate and oxygen flow rate areused to compute bath carbon loss during the refining process. These prior art processes fail to giveaccurate control, of the process because of the inaccurate knowledge of the start carbon in the total charge materials and the erratic natureof the reactions in the furnace. p

It is the primary object or this invention toprovide a process for accurately controlling the refining of amolten ferrous bath in a basic oxygen furnace based on the carb'on analysis of a sample of the molten ferrous bath obtained, preferably, while the vessel is in an upright position, and after a "desired portion of'an initially determined approximate total volume of oxygen has'been injected into the bath.

.It is a further objection of this invention to provide a process for controlling the refining of a ferrous bath in a basic oxygen furnace which requires only one interruption of the refining process.

Patented Nov. 24, 1970 'ice SUMMARY OF THE INVENTION Broadly, the process for controlling the refining of a molten ferrous bath in a basic oxygen furnace includes determining an initial approximate total volume of gas eous oxygen required to obtain a desired final carbon content of the molten ferrous bath, injecting the bath with a portion of the initial approximate total volume of oxygen to partially refine the bath, interrupting the oxygen injection to determine the carbon content of the bath, determining a specific volume of oxygen based on the carbon content of the bath at this point to obtain the desired final carbon content and injecting the bath with the specific volume of oxygen so determined.

The figure is a plot of the relationship between carbon and oxygen and the rate of removal of carbon by oxygen.

PREFERRED EMBODIMENT OF THE INVENTION We have found a process for accurately controlling the refining of a molten ferrous bath in a basic oxygen furnace which is not dependent upon accurate knowledge of the carbon content of the molten metal and solid scrap materials charged into the furnace nor upon consistent furnace reactions in the bath during the initial refining stages. The process, however, is dependent upon accurate knowledge of the carbon content of the molten ferrous bath at some point during the refining period after a major portion of the oxygen has been injected therein but prior to injecting substantially all of the oxygen therein.

* The desired point is hereinafter referred to as the P-I or fining of a molten ferrous bath in a basic oxygen furnace.

The curve was determined by plotting over points representing the carbon content of a molten ferrous bath at theprocess-interrupt point and the volume of oxygen required to reduce the process-interrupt percent carbon toa desired final percent carbon content. The desired final percent carbon content of the bath at final turn-down will hereinafter be referred to as the FTD%C or final turn-down percent carbon. In order to obtain the bestfit curve representative of the plotted points and to obtain the most accurate control in the refining of the bath it was necessary to develop a mathematical formula which could be solved for the volume of oxygen required to remove suflicient carbon after the process-interrupt percent carbon was determined to obtain the final turn-down percent carbon. As shown by the curve A-A', the rate of carbon removal during the initial refining stage, section 1 of the curve A-A, is a linear function of the volume of oxygen injected therein. During the latter refining stages, section 2 of the curve A- the rate of carbon removal is a non-linear function of the volume of oxygen injected therein. From this knowledge it was possible to obtain the following equation:

signed numerical values by computer analysis. The as signed numerical values are such that the sum of the squares of the differences between the observed values of the variable desired final turn-down percent carbon and the values of desired final turn-down percent carbon computed by the equation from corresponding observed values of the variable process-interrupt percent carbon and the variable is a minimum.

PI%C is the percent carbon by weight in the molten ferrous bath at the process-interrupt point.

FTD%C is the percent carbon by weight desired in the refined molten ferrous bath at final turn-down.

O is the volume of oxygen in cubic feet required to be injected into the molten ferrous bath after processinterrupt point to remove sufficient carbon to obtain the desired final carbon content in the refined molten ferrous bath.

The above equation is not susceptible to solution by commonly used regression analysis methods. It is first necessary to use a non-linear least square curve fitting computer program as devised by George Strubel for users of a 1620 computer and to apply a subroutine thereto. This program is available from International Business Machines Inc. Library for computer program. The subroutine may be a Newtonian iteration wherein the values of the desired final turn-down percent carbon may be computed from the above equation for observed values of oxygen (0 and the process-interrupt percent carbon using assumed numerical values for the constants A, B and E.

The use of the above described mathematical procedure resulted in the determination of assigned numerical values of 105,700, 2,425,000 and -l6.09 for A, B and E respectively. It will be understood that each basic oxygen furnace used in the industry has variable operating characteristics which will influence the numerical values of said constants A, B and E. Therefore, these numerical values must be determined for each furnace by conducting a statistically significant number of experimental tests as outlined above. By this above approach it was found that, unlike previous processes, an accurate knowledge of the carbon content of the aforementioned charged materials was not necessary. It is only necessary to know the approximate carbon content of the charged materials. Since the approximate carbon content is known and the desired final turn-down percent carbon FTD%C is known, the approximate total volume of oxygen required to refine the bath to thereby obtain the desired final turndown percent carbon may be calculated by any one of several known methods which need not be described. It has been found that if about 60% to about 80% and, preferably, 65% to 70% of the approximate total volume of oxygen so calculated is injected into the bath and the process is interrupted at this point to obtain the processinterrupt percent carbon, the values of the desired final turn-down percent carbon and process-interrupt percent carbon may be substituted into the above equation and a specific volume of oxygen required to reduce the process-interrupt percent carbon to the desired final turndown percent carbon obtained. The specific volume of oxygen is then injected into the bath to remove sufficient carbon therefrom to obtain the desired final turn-down percent carbon.

In the practice of the invention, molten metal and solid metallic scrap are charged into the furnace. The oxygen injection is started to thereby melt the solid charge materials and to start refining the molten ferrous bath thus formed. Afer a portion about 60% to about 80% of the calculated initial approximate volume of gaseous oxygen has been injected into the bath, the refining process is interrupted and a sample of the molten ferrous bath is removed from the furnace, preferably, while the furnace is in an upright position. The sample of the bath is removed from the furnace and is analyzed for its carbon content. After the sample has been removed from the furnace, oxygen injection of the bath is continued.

Since the desired final carbon content is known, the carbon content which must be removed from the bath after the process-interrupt point to achieve the desired final carbon content is also known. A specific volume of gaseous oxygen required to remove the necessary carbon to obtain the desired final turn-down percent carbon is determined by solving the aforementioned equation.

The specific volume of oxygen so determined is injected into the molten ferrous bath to complete the refining thereof. The actual total final volume of oxygen which must be injected into the molten ferrous bath is the sum of the volume injected prior to the process-interrupt point and the specific volume injected after the process-interrupt point. The actual total final volume of oxygen may be more or less than the initial approximate volume as originally calculated since the actual total final volume of gaseous oxygen required is determined by an accurate carbon analysis of the molten ferrous bath at the processinterrupt point.

A sample of the molten ferrous bath is obtained by the operator at the completion of the refining process to determine the amount of additions, for example, ferrosilicon or ferromanganese which must be made to the refined bath to obtain the desired final analysis.

In a specific example of the process, a 279 ton heat of steel having a desired final carbon content of 0.12% was made by injecting a charge of hot metal and scrap having an approximate carbon content of 3.62%. It was determined that an initial approximate volume of 497,500 cubic feet of oxygen would be required to obtain the desired final carbon content. The charge was injected with 320,000 cubic feet of substantially pure gaseous oxygen, which was 64.3% of the initial approximate volume of oxygen calculated to obtain the desired final carbon. The oxygen injection was interrupted at this point in the refining process and a sample of the molten bath was ob tained while the furnace was in an upright position. Analysis of the sample showed the crabon content of the bath to be 1.42%. To obtain the desired final carbon content of 0.12%, it was necessary to remove 1.30% carbon from the bath. By substituting for the process-interrupt percent carbon 1.42% carbon and for the desired final turn-down percent carbon 0.12% carbon in the formula previously described, it was found that a specific volume of 159,000 cubic feet of oxygen would now be required to be injected into the molten ferrous bath to obtain the desired 0.12% carbon in the finished bath. After the molten bath had been injected with the specific volume of 159,000 cubic feet of oxygen, the injection was stopped. The oxygen lance was raised out of the vessel and a sample of the bath was obtained for check analysis. The sample was found to have a carbon content of 0.11%. The bath was tapped into' a ladle where the necessary deoxidizers and alloying agents were added to the bath.

In another example of the invention, a 293 ton heat of steel having a desired final carbon content of 0.23%, was made by injecting a charge containing approximately 3.63% carbon. It had been calculated that an initial approximate volume of 500,200 cubic feet of oxygen would be required to obtain the desired final carbon content of 0.23%. The bath was injected with 320,000 cubic feet of oxygen when the process was interrupted to obtain a sample of the molten ferrous 'bath for carbon analysis. This volume of oxygen was 63.9% of the initial approximate volume required. The sample analyzed at 1.56% carbon. By substituting the necessary process-interrupt percent carbon and desired final turn-down percent carbon into the previously described formula, it was determined that a specific volume of 144,000 cubic feet of oxygen would now be required to remove 1.33% carbon to thereby obtain the desired final carbon content of 0.23 After the specific volume of 144,000 cubic feet of oxygen was injected into the molten bath the injection was stopped and a sample of the bath was obtained for check analysis. The bath was found to have a carbon content of 0.24%. The bath was tapped into a ladle where the necessary deoxidizers and alloying reagents were added to the bath.

It should be understood in this specification that wherever percentages are referred to such percentages are by weight except when referring to oxygen which percentages are by volume.

We claim:

1. A method for controlling the refining of a molten ferrous bath in a basic oxygen furnace to obtain a desired final carbon content comprising:

(a) charging metallic scrap and hot metal into the furnace,

(b) determining the approximate total volume of oxygen required to reduce the carbon of the charged materials to a desired final carbon content,

(c) partially refining the bath by injection of gaseous oxygen,

(d) interrupting the refining process after a volume of oxygen equivalent to 60% to 80% of the volume determined in step (b) has been injected into the bath,

(e) determining the carbon content of the partially refined bath at the process-interrupt point,

(t) determining the specific volume of oxygen now required to obtain the desired final carbon content according to the formula:

References Cited UNITED STATES PATENTS 3,181,343 5/1965 Fillon 75-60 X 3,236,630 2/1966 Stephan 75-60 3,372,023 3/1968 Krainer et al. 75-60 3,377,158 4/1968 Meyer et al. 75--60 3,463,005 8/1969 Hance 73-425.4

L. DEWAYNE RUTLEDGE, Primary Examiner G. K. WHITE, Assistant Examiner 

